1. Field of the Invention
The present invention relates to wireless communication and, more particularly, to a method and apparatus for transmitting channel information in a wireless communication system.
2. Discussion of the Related Art
A 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) system (i.e., the improvement of a Universal Mobile Telecommunications System (UMTS)) is introduced as the 3GPP release 8. The 3GPP LTE system uses Orthogonal Frequency Division Multiple Access (OFDMA) in downlink and uses Single Carrier-Frequency Division Multiple Access (SC-FDMA) in uplink. Multiple Input Multiple Output (MIMO) having a maximum of 4 antennas is adopted. Recently, a discussion on 3GPP LTE-Advanced (LTE-A) which is the evolution of 3GPP LTE is in progress.
In the next-generation wireless communication system, in order to improve the quality of service of a user placed at the boundary of a cell, cooperative multiple point transmission and reception (CoMP) or multi-cell Multiple Input Multiple Output (MIMO) is being introduced. Adjacent cells cooperate with each other to reduce interference or increase the intensity of a signal for a user placed at the boundary of a cell.
According to paragraph 7 of 3GPP TS 36.213 V8.7.0 (2009-05) “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures (Release 8)”, a mobile station feeds channel information back to a base station. The channel information includes a Precoding Matrix Indicator (PMI), a Channel Quality Indicator (CQI), a Rank Indicator (RI), etc. The RI indicates a rank calculated by a mobile station in the state where Single User-MIMO (SU-MIMO) is assumed. The rank means the number of multiplexed streams or the number of layers in the spatial domain.
If Multi-User-MIMO (MU-MIMO) or CoMP is introduced, the existing feedback method may have several problems.
First, system performance may be degraded owing to the inaccuracy of a rank. The rank is determined assuming SU-MIMO (here, the rank is called an SU-MIMO rank), but an optimum rank may be different from the SU-MIMO rank if the CoMP or the MU-MIMO is applied. For example, if a Signal-to-Interference plus Noise Ratio (SINR) is increased through the CoMP, an optimum rank may have a rank higher than the SU-MIMO rank. However, a base station may not fully obtain an optimum performance gain obtainable from the CoMP by adjusting only a Modulation and Coding Scheme (MCS) under the SU-MIMO rank.
For example, since the SINR is greatly increased in joint transmission where cooperative cells transmit data at the same time, more streams may be multiplexed by using a higher rank in order to achieve a higher data transfer rate. In general, it is preferred that a rank lower than the SU-MIMO rank be used because a plurality of user streams is multiplexed in MU-MIMO.
It is important to set an optimum feedback in the CoMP and/or MU-MIMO because a CQI and a PMI are determined according to the rank.
Second, the efficiency of radio resources used for the feedback of channel information may be degraded. A base station allocates radio resources on the basis of the highest rank for the feedback of channel information if the base station does not know the rank of a mobile station. If a rank actually used by the mobile station for feedback is small, however, resources may be wasted because only some of the allocated radio resources are used for the channel information feedback.
In the CoMP, channel information about not only a serving cell, but also adjacent cells participating in cooperation has to be fed back. Thus, if feedback overhead is increased, the amount of wasted radio resources may be further increased. If radio resources are allocated assuming a maximum feedback payload size, the amount of wasted radio resources may be increased in proportion to the number of cells participating in cooperation.
There is a need for a more efficient channel information feedback scheme in a multiple antenna system or a multiple cell cooperation system.